1. Field of the Invention
The present invention is generally in the field of electronic communications circuits and systems. More specifically, the present invention is in the field of communications filters.
2. Background Art
Notch filters are typically used in satellite receiving systems to notch out a specific frequency range. Satellite receiving systems typically utilize a down-converter and a local oscillator to mix a high frequency input signal down to an intermediate frequency (“IF”) signal, which is then amplified by a low noise amplifier. Additionally reducing the overall power at the amplifier input by using a notch filter reduces the level of the second and third order intermodulation products produced by the amplifier after the notch filter thereby improving the signal to noise and distortion ratio (SINAD) of the overall satellite receiver system.
Amplification of the low frequencies in a satellite frequency band can produce second harmonic frequencies that interfere with the high frequencies in the same frequency band. These are commonly called second order intermodulation products of the amplifier and are due to nonlinearities of the amplifier which are specified by the IP2 performance of the amplifier. For example, consider a satellite receiving system that is to tune and amplify a satellite frequency band of 950 MHz to 2150 MHz (approximately 1 to 2 GHz). The 950 MHz to 1075 MHz (approximately 1 GHz) band can produce second harmonic frequencies that interfere with the 1900 MHz to 2150 MHz (approximately 2 GHz) band. Thus, tuning and amplification performance within the 1900 MHz to 2150 MHz band can suffer as a result of signal interference from the undesired second harmonics of the 950 MHz to 1075 MHz band.
Conversely, in a direct conversion receiver, the second harmonic frequencies of a local oscillator can mix with the high frequencies of a satellite frequency band to result in lower frequencies that interfere with the low frequencies within that satellite frequency band. For example, consider a satellite receiving system that is to tune and amplify a satellite frequency band of 950 MHz to 2150 MHz. The second harmonic frequency of a local oscillator can mix with the 1900 MHz to 2150 MHz band to result in lower frequencies that interfere with the 950 MHz to 1075 MHz band. Thus, tuning and amplification performance within the 950 MHz to 1075 MHz band can suffer as a result of signal interference from the undesired second harmonics of the local oscillator mixing with the 1900 MHz to 2150 MHz band.
Conventional notch filters to filter out a specific narrow range of frequencies in satellite receiving systems, e.g. either the 1 GHz or the 2 GHz frequency range, have utilized cumbersome inductance-capacitance filters that are expensive and require large amount of circuitry. Moreover, the conventional notch filters do not switch from notching out one range of frequency to another (for example from 1 GHz to 2 GHz and vice versa) with symmetry and effectiveness. There is thus a need in the art for effectively reducing signal interference in a satellite receiving system without the shortcomings of the conventional notch filters.